Study On Carbon-based Catalyst For Zinc-air Battery With Ce Single-atom-sites

Zinc-air batteries（ZAB）are a new energy conversion technology that is clean,environmentally friendly,economically efficient and with high energy density,but the oxygen reduction reaction（ORR）on the cathode suffers from slow kinetics and complex reaction paths.Therefore,the design and synthesis of efficient ORR electrocatalysts has been a hot research topic in ZAB.Currently,the most widely used catalyst in ORR is the commercial Pt/C catalyst,but the development and commercial application of ZAB are limited by the disadvantages of scarce Pt resources,high price,unstability and easy poisoning.In recent years,non-Pt Fe-N-C materials have been considered as one of the important alternatives to noble metal catalysts,but their poor stability due to severe Fenton（Fenton）reaction cannot achieve the goal of long-time charge and discharge of ZAB.For this reason,the rational design of other non-platinum ORR catalysts with high activity and stability is crucial to achieve rapid commercial applications of ZAB.In this thesis,two types of rare-earth single-atom Ce-based catalytic materials were synthesized mainly by using supramolecular gel-assisted pyrolysis technique and nanopore-limited atomic coordination strategy.And we further investigated the change laws affecting the morphological structure,active sites and electrochemical activity and stability of the catalysts by exploring the atomic-doping mode,material composition and pyrolysis process.Besides,we tested and comprehensively evaluated the performance of ZAB assembled with above-mentioned Ce-based catalysts.The main works are carried out as follows.（1）A supramolecular gel-assisted pyrolysis（SGP）method with the self-assembly of nitrate ions with melamine was proposed to synthesize single-atom Ce-based oxygen reduction catalysts（CeO₂/SACe-N-C）with a three-dimensional porous carbon nanonetwork structure.The supramolecular gels can effectively immobilize Ce ions and further form Ce monoatomic sites at high temperature（total Ce content:8.8 wt.%）.X-ray diffraction and X-ray absorption spectroscopy measurements demonstrate that the chemical forms of Ce in the catalysts were mainly Ce-N₄O₄ structures and CeO₂ nanoparticles.the CeO₂/SACe-N-C catalysts in 0.1 M KOH solution exhibited slightly lower ORR catalytic activity than the Pt/C.The assembled primary ZAB test using CeO₂/SACe-N-C shows a high energy density of 830 Wh/kg _Zn,and the secondary charge/discharge ZAB test also indicates a relatively small charge/discharge potential difference and good long-term cycling stability.DFT theoretical calculations show that the d-band centers of Ce atoms in Ce-N₄O₄|CeO₂ are closer to the Fermi energy level than in CeO₂ nanoparticles（E_F）,and the Ce-N₄O₄|CeO₂ coupling site is more favorable for the activation of adsorbate O₂ and optimization of the ORR intermediate reaction pathway,which in turn leads to the CeO₂/SACe-N-C catalysts with relatively high oxygen reduction catalytic performance.To the best of our knowledge,very few studies have been reported on the use of the SGP method to design catalytic catalysts with Ce-N₄O₄|CeO₂ coupling sites to enhance ORR catalytic properties.This work will provide new ideas and methods for the subsequent development of various rare-earth single-atom ORR catalysts with a high density of coupling catalytic sites.（2）In order to reduce the formation of Ce nanoparticles as well as to increase the density of single-atom Ce-N_x sites,a new strategy for nanopore-confined atomic coordination was further proposed to the synthesis of carbon-based catalysts containing abundant single-atom Ce sites（Ce SA/MC）in hierarchically ordered nitrogen-doped mesoporous materials.It was found that the Ce SA/MC catalysts obtained by in situ the pyrolysis of Ce complexes in SBA-15 mesoporous templates have exhibited ultra-high specific surface area（1108.7 m²/g）and large pore volume（0.83 cm³/g）.The nano-confinement effect can effectively reduce the thermal loss of Ce and N atoms,further increasing the Ce single-atom loading（0.28 wt.%）and active site density.Ce SA/MC catalysts have exhibited ORR activity comparable to Pt/C in alkaline solutions(E_1/2=0.845 V vs.RHE),and their assembled primary ZAB possesses a high energy density of 924 Wh/kg _Zn and secondary charge/discharge ZAB tests also exhibit excellent long-term stability and small charge/discharge potential difference（ΔE=82.6 m V）.DFT calculations showed that the Ce SA/MC catalyst can facilitate the chemisorption and activation of the adsorbed species,and the energy barrier of Ce SA/MC in the rate-determining step（0.12 e V）was lower than that of the nitrogen-doped carbon material without Ce atoms（0.35 e V）,indicating that the relevant ORR intermediate reaction pathway was optimized to some extent,which helped to further improve the resulting Ce-based catalyst.To the best of our knowledge,it is the first time to use a nanopore-confined atomic coordination method to synthesize carbon-based single-atom catalysts with an abundant Ce-N₄ structure,which will provide a technical reference for designing various mesoporous structure-based rare-earth SA catalysts and also provide a new pathway for the subsequent development of non-platinum ORR catalysts for fuel cells.